Psy-Kosh, kin selection means that you help those who are closely related to you, which helps spread your genes since those who are related to you are more likely to carry your genes. It's beneficial to breed, since (in sexual reproduction) your children share 50% of your genes. It's likewise beneficial to help your siblings, since they, too, share 50% of your genes. Hamilton's rule states that an allele for altruistic behavior will spread if the behavior it causes obeys the equation Br > C, where r is the relatedness between the actor and recipient, B is the benefit to the recipient and C is the cost to the actor, with B and C measured in surviving offspring. In other words, in order for altruism to spread, the behavior must spread your genes more than investing those resources to your own offspring would.

This is different from group selection, for you are only helping those who are very closely related to you (and therefore are likely to have the same allele), while in group selection you'd also be helping ones who didn't carry your genes and therefore harming yourself. Think of it this way - in order for an allele to spread, it must give its bearer a greater fitness than any alternative ones. An allele which helps everyone equally doesn't do that. Kin selection, on the other hand, does give a greater fitness to those who are more likely to carry the allele - in the form of help from their relatives.

Wiseman, it's true that the probability of survival for each cub depends on how much food it gets, which in turn depends on how many cubs the food has to be divided with. And yes, in extreme situations where the food is very scarce, having too many cubs means that they will likely all die - but I'm not convinced that the slow-breeder advantage during times of extreme scarcity will be enough to keep their numbers up during times of less scarcity, when they are being outbred. I'll try to quickly program a rough model for it to see what sort of numbers it produces (though I'm no evolutionary biologist).